WASHINGTON - U.S. President Barack Obama heads to Copenhagen on Thursday to help secure a U.N. climate pact, staking his credibility on an as yet elusive deal that has ramifications for him at home and on the world stage.

Obama is expected to arrive in the Danish capital on Friday morning, joining about 120 other world leaders to finish a complicated process of reaching a political agreement to reduce greenhouse gas emissions and fight global warming.

The time is short and the stakes are high. With his top domestic priority of healthcare reform legislation percolating in Washington, the president plans to stay in Copenhagen less than a day.

That may or may not be enough time to overcome persistent disagreements between developed and developing nations that have marred two weeks of talks, but Obama's presence and contribution could be a potential deal-maker.

The United States has proposed to cut its greenhouse gas emissions in the range of 17 percent by 2020 compared with 2005 levels. That corresponds to a 3 percent reduction from 1990 levels, the baseline used by the European Union and others.

Obama is unlikely to propose a more aggressive emissions reduction target, which many countries have demanded. His goals are based on a bill that passed the House of Representatives but has yet to go through the Senate before it can become law.

Still, White House spokesman Robert Gibbs said Obama hoped to help break a deadlock around outstanding issues surrounding developed countries' emissions targets and disagreements about financial support for poor countries dealing with climate change.

I think leaders representing developing and developed nations all over the world coming to Copenhagen gives ... an opportunity for some of those issues to be resolved and a breakthrough to happen, Gibbs said on Wednesday.

The president is ... hopeful that his presence can help that, and hopeful that, again, we leave Copenhagen with a strong operational agreement, even as we work towards something even stronger in the future.

Environmentalists say Obama could turn the talks around by pledging his strong support for the Senate climate bill, which has a more aggressive 20 percent emissions reduction target, and by putting his full efforts into the issue once healthcare reform is finished.

He could also ease conflicts over funding by promising to ask Congress for more money in the U.S. budget for fiscal 2011 to help poor countries adapt to climate change.

His visit is fraught with risks. If the president, a Democrat, puts a more aggressive offer on the table, he could face criticism from Republicans who charge the United States is going too far without getting enough in return from big developing economies such as India and China.

If he is more cautious and the talks end up faltering, he would be connected to that failure and his efforts to pass domestic climate change legislation could suffer along with his credibility among other international leaders.

He's sort of damned if he does, damned if he doesn't, and (so) he might as well do the thing that's right, said Alden Meyer, director of strategy and policy at the Union of Concerned Scientists, urging Obama to push the talks forward.

This is the kind of thing that, if you think about it, he ran for president to do. The kind of thing he got awarded his Nobel Prize because of the potential to do, Meyer said.

Obama has been making phone calls to other world leaders this week to discuss the process before his arrival.

There is some speculation Obama would also sign an updated pact with Russian President Dmitry Medvedev to reduce nuclear arms stockpiles during his short European trip, but the White House played down the chances a deal on that issue would be reached in time.
(Editing by Peter Cooney)